Bicycles/Maintenance and Repair/Appendix/Moulton/Mk III, AM & ATB/APB rear triangle

Application
The AM rear triangle (also called rear fork) suspension is used on the AM series (1982-current), also with detail differences on AM-ATB and APB. It follows the same general arrangement originally developed for the Mk III rear fork, but at a reduced weight. A number of running changes have been made over the years.

Principle of Operation
The AM rear triangle suspension geometry is a simple trailing arm, with the pivot behind the bottom bracket. The rubber suspension spring is in compression, located at (or near) the height of the top of the main frame. The motion ratio is approximately 2:1 wheel_travel:spring_travel and nearly linear. The suspension incorporates a rising spring rate which is designed into the rubber spring.

Component Identification
The rear triangle (rear fork) is a fabricated unit including rear dropouts for the axle, derailleur hanger (on certain models), brake bridge (except on AM2 version), a forward/lower cross tube that carries the pivot bushings and the upper attachment point for the spring. The lower and upper (diagonal) main tubes are called chain stays and seat stays, same as for a diamond frame bicycle. A small "X-brace" is located vertically in front of the rear tire, this stabilizes the structure and also provides a mounting point for the rear mudguard.

Variations --


 * MkIII is assembled with cast lugs, considerably heavier than AM brazed rear forks.


 * AM versions have used several different dropout designs, all made from stainless steel plate:
 * 1) Original, horizontal dropouts, no hanger, for AM2 model (later AM5 and other hub gear models).  Note that these rear triangles do not have a brake bridge, while there is a tube in that location, it is thin wall and not suitable for mounting a caliper brake.
 * 2) Original, vertical dropouts, long derailleur hanger, designed for the geometry of Suntour touring rear mechs of that era such as Cyclone M-II.
 * 3) Vertical dropouts, mid-length hanger, designed for later Suntour such as Sprint (or Superbe Pro with max cog 24T).
 * 4) Similar to above, but no provision for rear mudguard -- for AM-SPEED.
 * 5) Slightly off-vertical dropouts, very short hanger, designed in conjunction with an engineer from Shimano, to work with Shimano indexed shifting.


 * ATB includes a height adjusting screw as part of the spring attachment. Tube count is reduced by making chain stays, seat stays and the forward vertical member from one bent tube.


 * APB is similar to ATB but without height adjuster.

Suspension bearings are plastic in MkIII and Oilite bronze in the more recent models.

The MkIII spring is a rubber squash ball with bonded in attachment bolts. The AM and related models use a cone spring -- rubber bonded between inner and outer cone-shaped aluminum shells (sometimes referred to as monosphere).

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Disassembly
Cut-paste error below -- replace the para below with Rear Suspension section!

The TSR Service Instruction Front Suspension (PDF) linked nearby includes instructions to disassemble the fork. As noted, it may be necessary to compress the suspension slightly to release pressure on the leading links--this can be done with a rope or strap looped around the stirrup and through the frame (or over the crown if fork removed from bike).

Oilite bearing replacement
Based on a Yahoo list thread from

Sourcing -- Oilite is available from bearing suppliers. The size

Installation -- Moulton dealers should be able to do this, but it is straightforward for anyone with DIY experience. Make a pressing tool from an 8mm (or 5/16 inch = 7.94mm) diameter bolt with smooth shank to match the ID of the DU bearing. Slide the new DU onto the bolt, and then slide the bolt into the old DU in the fork end. Support the other side of the fork end with a suitable tube (eg, a 14mm socket) and then press or tap the old DU out and the new DU in. Use a drop of oil between DU outer shell and fork end. If the bolt is long enough to pass through the support (14mm socket), a nut and washer will make a nice controlled way to pull the new DU into place. One list member used a drill press as an arbor press. Center the DU bearing in the fork or stirrup.

Caution -- Don't tap on the end of the DU, the thin shell is easily distorted, always put the bolt through first and tap on the head of the bolt. If you are nervous, buy a spare DU bearing in case you ruin one before you get it right.

Lubrication -- per the DU literature the preferred lubrication is none (dry). Acceptable operation can also be had with oil, grease or water. This writer recommends inspecting the journals to make sure they are clean and polished smooth. Repair/replace any journals that are damaged and then assemble dry. Note that the journals are bonded onto more recent leading links and are not easily removed--worst case may require new leading links.

Choice of Spring
When the fork is apart, take a close look at the spring. After considerable mileage, it is common for the spring wire to be worn on the outside making the (originally circular cross section) wire into a "D" shape. This severely weakens the spring. Riders of average weight will likely experience unpleasant suspension bottoming.

Candidates for a stiffer spring are heavier riders, and also riders who lean forward on the handlebars (front wheel carries more weight).

Reassembly
The TSR Service Instruction xxx linked nearby includes instructions to reassemble the fork. At time of writing (2020) TSR (and possibly other new production) use a 12.9 high tensile bolt to work with Nord-Lock® lock washers. The recommended torque for the 12.9 high strength rear pivot bolt is 30Nm.

Adjustment
Once properly assembled there are no further adjustments on the AM rear suspension. It is important that the pivot bolt stays tight, worth a check from time to time. If it loosens, the journal (crush tube) will be free to move and the bolt holes in the frame ears will "ovalize"--in extreme cases requiring new ears at the factory.

Some dealers may have access to rubber cone springs with slightly different stiffness (due to manufacturing variations in rubber compound or curing time) but the standard spring works well for a wide range of rider weights.